"Agonizing" is a participle, a verb form that is being used as an adjective in this sentence. There are present tense participles and past tense participles, and both function as adjectives. One way to test for this is to take the participle and see if you can use it to describe the noun. For example, in your sentence, it is possible to say,
There was an agonized wait for the decision.
Here is an example of a past participle that functions as an adjective:
The crowd was shaken by the explosion.
We are able to say,
The shaken crowd waited for another explosion.